1. Field of the Invention
The present invention relates to a liquid crystal display device, and in particular to a liquid crystal display device having excellent viewing angle characteristics.
2. Description of the Related Art
Liquid crystal display devices incorporating nematic liquid crystal display devices, in which a nematic liquid crystal is interposed between a pair of substrates, have conventionally been generally used for numeral segment type display devices such as clocks and calculators. Active switching elements such as TFTs (Thin Film Transistors) are formed on a light transmitting substrate so as to function as a switching means for selectively driving pixel electrodes for applying a driving voltage to the liquid crystal. On a counter substrate provided so as to oppose the light transmitting substrate, a plurality of minute color filter layers having colored regions of red, green, and blue are provided as passive elements of a color display means.
Examples of such liquid crystal display devices, as categorized based on the twist angle of the liquid crystal, are: (a) active type, twisted nematic (hereinafter referred to as "TN") liquid crystal display system in which nematic liquid crystal molecules are aligned so as to be twisted by 90.degree.; and (b) multiplex type, super twisted nematic (hereinafter referred to as "STN") liquid crystal display system in which nematic liquid crystal molecules having a twist angle of 90.degree. or more are used so as to attain steep voltage-light transmittance characteristics, (the voltage being applied to the liquid crystal).
In general, the active type TN liquid crystal display system is further categorized into: (a-i) a normally black system where a pair of polarizing plates are disposed in such a manner that the polarizing directions thereof are parallel to each other, so that a black image is displayed when no voltage is applied to the liquid crystal layer (i.e., in an OFF state); and (a-ii) a normally white system where a pair of polarizing plates are disposed in such a manner that the polarizing directions thereof are perpendicular to each other, so that a white image is displayed in an OFF state. Among the two systems, the (a-ii) normally white system is considered to be more promising in terms of color reproductivity and viewing angle dependence of display.
FIGS. 7A to 7C each show a cross section of a conventional TN type liquid crystal display device 1. The problems associated with a conventional TN type liquid crystal display device will be described with reference to FIGS. 7A to 7C.
The display device 1 includes a pair of glass substrates 2 and 3 and a liquid crystal layer 4 interposed between the glass substrates 2 and 3. As shown in FIG. 7A, liquid crystal molecules 5 that constitute the liquid crystal layer 4 are aligned in such a manner that the longitudinal axes thereof are twisted by 90.degree. between the substrates 2 and 3.
In the conventional TN type liquid crystal display device 1 of the above configuration, if a voltage is applied to the liquid crystal layer 4 with the use of a driving circuit 6 while light is incident on the liquid crystal display device 1, so that an electric field in the vicinity of the liquid crystal layer 4 is turned on and off, the liquid crystal molecules 5 stand uniformly in the same direction when a voltage is applied to the liquid crystal layer 4 as shown in FIGS. 7B to 7C. As a result, the liquid crystal display device 1 has limited viewing angle characteristics.
In the TN type liquid crystal display device 1 shown in FIGS. 7A to 7C, liquid crystal molecules, which originally have refractive index anisotropy, are aligned at a certain angle with respect to the pair of substrates 2 and 3, on whose surface electrodes are formed, so that the contrast of a displayed image depends on a viewing angle at which a viewer observes the liquid crystal display device. In other words, the TN type liquid crystal display device 1 has a large viewing angle dependence. In particular, as the viewing angle becomes large with respect to a normal axis direction to the display, toward a direction where the display contrast improves, the black and white regions of the image suddenly reverse at a point when the viewing angle exceeds a certain value. This is well known as a "reverse phenomenon".
In order to improve the viewing angle dependence, attempts have been made to compensate for the phase difference between an ordinary light component and an extraordinary light component by incorporating a film-like phase difference plate between the substrates and the polarizing plates, the phase difference plate being such that one of the directions of the principal refractive indices of index ellipsoids, e.g., a polymer, is parallel to the normal axis of the display. However, the above-mentioned reverse phenomenon can only be improved to a certain extent, even by using such a phase difference plate.
Other methods that have been proposed for improving the viewing angle dependence of a TN type liquid crystal display device are: a pixel dividing method (disclosed in Japanese Laid-Open Patent Publication No. 57-186735), a rubbing angle optimization method (disclosed in Japanese Laid-Open Patent Publication No. 4-221935), and a non-rubbing method (Japanese Patent Publication No. 3-14162). However, each of the above methods has inherent problems.
The pixel dividing method, for example, can eliminate the reverse phenomenon and viewing angle dependence along the vertical direction. However, this method has problems in that: the display contrast lowers; a black image appears grey when the viewing angle is tilted from the normal axis direction, i.e., a discoloration problem; and the liquid crystal display device has viewing angle dependence along a horizontal direction (i.e., from left to right) and/or a vertical direction (i.e., from top to bottom). The rubbing angle optimization method, although capable of improving the reverse phenomenon between white and black images, has problems in that a displayed image can appear properly in only a small range of viewing angles in a direction opposite to a direction along which a reverse phenomenon may occur, and that the display contrast when the liquid crystal display device is viewed at the normal axis direction decreases. The non-rubbing method has a problem in that disclination lines remain conspicuous.
Still another method for improving the viewing angle dependence is a method using an LB film produced by the Langmuir-Blodgett's technique, disclosed in Japanese Laid-Open Patent Publication No. 3-293324. According to this method, molecular chains (main chains) of the LB film are generally uniformly aligned along a predetermined direction so that liquid crystal molecules are aligned along the direction of the molecular chains of the LB film. For an LB film, a stereoregular polymer is preferably used. A stereoregular polymer is a polymer having regularity with respect to its three-dimensional structure on the atomic level. Accordingly, a liquid crystal display device incorporating an LB film has viewing dependence such that the contrast of the liquid crystal display device varies depending on the viewing angle along vertical and horizontal directions at which the liquid crystal display device is observed. Therefore, this method has a problem in that the abovementioned image displaying defects may occur, thereby degrading the quality of the displayed image.
In general, the above-mentioned (b) multiplex type STN liquid crystal display system is further categorized, depending on the kind of optical phase difference elements used for the liquid crystal display device in order to ensure proper display of white and black images, into: (b-i) a two-layer type, double super twisted nematic (hereinafter referred to as "DSTN") incorporating a multistructure including a liquid crystal display device for a display purpose and a liquid crystal display device for an optical compensation purpose in which liquid crystal molecules are aligned in a twisted manner at a twist angle in the opposite direction of a twist angle of the liquid crystal display device for a display purpose; and (b-ii) a film-equipped type, liquid crystal display system, according to which a film having optical anisotropy is attached to a liquid crystal display device. Among the two systems, the (b-ii) film-equipped type liquid crystal display device system is considered to be more promising in terms of reduction of the mass and production cost of the liquid crystal display device. However, either of the liquid crystal display systems of (b-i) and (b-ii) has a problem of a characteristic coloration phenomenon of the STN liquid crystal.